Device and method of correcting exposure defects in photolithography

ABSTRACT

A corrective filter for use in an optical system to correct a defect in a reticle and/or pellicle. The corrective filter may be positioned between a light source and the reticle, between the reticle and a wafer, or in combination with the reticle and/or pellicle. The invention provides a method of characterizing the optical properties of the corrective filter.

RELATED APPLICATIONS

[0001] This application is a continuation of, and hereby claims priorityto and incorporates by reference in its entirety co-pending U.S. patentapplication Ser. No. 09/332,856, filed Jun. 14, 1999, and entitled“DEVICE AND METHOD OF CORRECTING EXPOSURE DEFECTS IN PHOTOLITHOGRAPHY.”

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to the correction of exposurevariations in an optical system. More particularly, this inventionrelates to correcting variations introduced by one or more components,such as a reticle or pellicle, into the exposure field of the opticalsystem.

[0004] 2. Description of the Related Art

[0005] Optical systems are widely used in the microelectronics industryto manufacture semiconductor wafers by a process known asphotolithography. Typically, a photolithography system comprises a lightsource configured to project light rays to a condenser lens. Thecondenser lens collimates the light rays towards a pellicle placedbefore (or after) a reticle. Typically, the reticle is an opticallyclear quartz substrate having a chrome pattern used to project an imageonto a portion of a photoresist-coated wafer. The pellicle is a verythin, transparent film which seals off the reticle surface from airborneparticulates and other forms of contamination. A projection lens isplaced after the reticle to receive and focus the light rays onto anexposure field on the wafer.

[0006] In designing such an optical system, any one of these componentsmay be vulnerable to manufacturing imperfections which, even if minutelysmall, may cause intolerable or unacceptable defects in the photoresistlayer of the wafer. For example, aberrations due to defects in one ormore lenses may include distortion, curvature of field, sphericalaberration, and astigmatism. Moreover, distortions may be due to defectsin the reticle that may be caused during manufacturing. For example,reticle defects may arise from impurities in the chrome, lack ofadhesion of the chrome to the reticle, variances in ion beam used toproduce chrome etching, or other similar defects. Reticle defects maycause intolerable or unacceptable variations in critical dimensions(CD's) in the exposure field. A CD represents the width or space ofcritical elements in an integrated circuit (IC).

[0007] Several attempts were made in the industry to compensate forgeneral defects in the optical system. For example, in U.S. Pat. No.5,640,233 issued to McArthur et al., a two-plate corrector is disclosedin a stepper configuration so that an image from a reticle plane isprojected to an ideal image at an object plane. Based on the premisethat depth of field correction made at the reticle plane inducesinsignificant distortion, McArthur describes placement of the two-platecorrector at the reticle plane to correct depth of field distortionscaused by the lens system. However, McArthur does not describe how tocorrect defects resulting from specific components, such as the reticleor pellicle.

[0008] To eliminate undesirable variances that result from defects inthe reticle, some manufacturers simply replace the defective reticlewith a new reticle. Other manufacturers resort to discarding wafershaving intolerable CD's caused by the defective reticle. In either case,a significant increase in manufacturing cost due to reticle defects hasbecome unavoidable.

[0009] Therefore, there is a need in the industry to compensate forindividual component defects without having to replace the component ordiscard any defective wafers resulting therefrom. The solution ofcorrecting such defects should be cost-effective and easy to implement.

SUMMARY OF THE INVENTION

[0010] To overcome the above-mentioned limitations, the inventionprovides a photolithography system having a light source configured toproject light onto an object. The system comprises a reticle positionedto receive the projected light from the light source, wherein thereticle includes a defect. The system further comprises a filterpositioned between the light source and the reticle. The filter has acorrective element that is geometrically related to the location of andconfigured to substantially correct the defect in the reticle. Inanother embodiment, the photolithography system comprises a reticlepositioned at a reticle distance from the light source to receive lighttherefrom, wherein the reticle includes a defect. The system furthercomprises a filter positioned at a filter distance from the light sourcethat is greater than the reticle distance. The filter is configured toreceive light passing through the reticle. The filter has a correctiveelement that is geometrically related to the location of and configuredto substantially correct the defect in the reticle.

[0011] In another embodiment, the invention provides a reticleconfigured for use in a photolithography system. The reticle comprises afirst plate having a predetermined pattern which includes a defect forprojection onto an exposure field. The reticle further comprises asecond plate attached to the first plate. The second plate comprises afilter having a corrective element that is geometrically related to thelocation of and configured to substantially correct the defect in thefirst plate. The invention also provides a method of correcting a defectin the reticle of the photolithography system. The method comprises thestep of obtaining at least one measurement of a feature in an exposedimage. The method further comprises the step of analyzing themeasurement in connection with an expected feature in the exposed image.The method further comprises the step of determining the opticalcharacteristics of a filter based on the analyzing step, the filterbeing suitable for substantially correcting the defect in the reticle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other aspects, features and advantages of theinvention will be better understood by referring to the followingdetailed description, which should be read in conjunction with theaccompanying drawings, in which:

[0013]FIG. 1 is a schematic diagram showing a typical optical systemhaving a defective reticle.

[0014]FIG. 2 is a schematic diagram of the optical system of FIG. 1having the corrective filter in accordance with the invention.

[0015]FIG. 3 is a perspective view of one embodiment of the correctivefilter built in combination with a defective reticle.

[0016]FIG. 4 is a flowchart describing the steps of determining thecharacteristics of the corrective filter of FIG. 2.

[0017]FIG. 5A is a ray diagram of a reticle having one or more defects.

[0018]FIG. 5B is a ray diagram of the reticle of FIG. 5A aftercorrection by the corrective filter in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] The following description is not to be taken in a limiting sense,but is made merely for the purpose of describing the general principlesof the invention. The scope of the invention should be determined withreference to the claims.

[0020]FIG. 1 is a schematic diagram showing typical physical componentsof an optical system having a defective reticle. A photolithographysystem 100 comprises a light source 110 configured to project light rays112 to a condenser lens 120. The condenser lens 120 collimates the lightrays towards a reticle 130 having a chrome pattern 132. As noted above,the reticle 130 may be an optically clear quartz substrate used toproject an image onto a surface. The reticle 130 is often covered by apellicle 140 to seal off the surface of the reticle 130 from airborneparticulates and other forms of contamination. As further describedbelow contaminants may interfere with the optical characteristics of thereticle 130 and, hence, distort the projection of the pattern 132. Thepellicle 140 is a very thin, transparent film which may not interferewith the optical characteristics of the reticle 130. The pellicle 140may also reduce the frequency of cleaning and extend the life time ofthe reticle 130.

[0021] The pellicle 140 passes the light rays 112 to a lens system 150.Although only one lens is shown in FIG. 1, the lens system 150 maycomprise one or more optical lenses. The lens system 150 focuses thelight rays 112 onto an exposure field, such as a photoresist layer 160.The photoresist layer 160 typically comprises a light-sensitive,substantially uniform, and thin film spread on a wafer 170. A desiredpattern may be formed by exposing the photoresist layer 160 to lightthat is masked with a predetermined pattern, e.g., the pattern 132 ofthe reticle 130.

[0022] As described above, when designing such an optical system, anyone of these components may be subject to manufacturing imperfections.More particularly, pattern distortions may result from defects causedduring manufacturing of the reticle 130 or pellicle 140. These defectsmay cause unacceptable variations in the pattern on the photoresistlayer 160. For example, due to a defect in the reticle 130, a point A ofthe pattern 132 may be projected as A′ at a location before its intendedprojected location on the photoresist layer 160. Similarly, a point B ofthe pattern 132 may be projected as B′ at a location after its intendedprojected location on the photoresist layer 160. Accordingly, any pointon the pattern 132 may deviate from its intended projected location dueto a distortion in virtually any geometric direction of the threedimensional space. As noted above, any of these pattern distortions mayvary the CD's in the wafer 170 to unacceptable levels, which includeCD's that are bridged, relatively small, scummed, or other defects whichare generally known for CD's in the art. Accordingly, it is desirable tocorrect these distortions without having to replace the reticle 130and/or pellicle 140.

[0023]FIG. 2 is a schematic diagram of the photolithography system 200having a corrective filter 210 in accordance with the invention. In oneembodiment, the corrective filter 210 may be placed at any positionbetween the light source 110 and reticle 130, as shown in FIG. 2. Inanother embodiment, the corrective filter 210 may be placed between thereticle 130 and photoresist layer 160, i.e., after the reticle 130. Itis desirable to place the corrective filter 210 at a location that doesnot interfere with heat dissipation characteristics of the lens system150. In some applications, the lens system 150 may heat up due to energyemanating from the light rays. Typically, the rise in temperature of thelens system 150 affects its optical properties and, consequently, causesa distorted projection of light onto the photoresist layer 160. Tocompensate for such optical distortion, the lens system 150 is oftencontrolled by an optical algorithm 220 which adjusts the opticalproperties of the lens system 150 as a function of temperature. Placingan object within a close proximity of the lens system 150 may interferewith its heat dissipation characteristics and cause the opticalalgorithm to erroneously compensate for heat variations. The distancebetween the lens system 150 and reticle 130 varies from onephotolithography system to another and, generally, is about 3-4 inches.Thus, it is desirable to place the corrective filter 210 at a minimumdistance of about 1 inch from the lens system 150. However, if themachinery or robotic equipment that places the corrective filter 210 isconfigurable to such distances, other distances may be acceptable solong as interference with heat dissipation characteristics of the lenssystem 150 is tolerable.

[0024] In another embodiment, the corrective filter 210 may be designedfor placement in close proximity to, or manufactured in combinationwith, the reticle 130. For example, the pellicle 140 may be designed tohave the corrective properties of the corrective filter 210 forattachment to the reticle 130. Alternatively, the corrective filter 210may be designed separately from the pellicle 140 and attached to thereticle 130 to form a substantially coherent unit. In such anembodiment, the reticle 130 effectively comprises a first plate thatcontains the predetermined pattern and a second plate attached to thefirst plate. Thus, the second plate includes the corrective filter 210.As demonstrated by these examples, there may be several designvariations and locations of the corrective filter 210 which will becomeapparent to one of ordinary skill in the art from this description.

[0025] With the corrective filter 210 positioned between the condenserlens 120 and reticle 130, the reticle 130 correctly projects the pattern132 onto the photoresist layer 160. More particularly, the correctivefilter 210 compensates for any variation in substantially all the pointsof the pattern 132. For example, the corrective filter 210 corrects theprojection of the point A of the pattern 132 so that its projection A′is located at the intended position on the photoresist layer 160.Similarly, the point B of the pattern 132 is corrected so that itsprojection B′ is located at the intended position on the photoresistlayer 160. As illustrated by the projection of the two points A and B,the corrective filter 210 corrects substantially all forms of opticaldistortions including reticle and pellicle defects due to myopia,hyperopia, and astigmatism.

[0026]FIG. 3 is a perspective view of one embodiment of the correctivefilter 210 built in combination with a defective reticle 130. As notedabove, the corrective filter 210 may be attached to the reticle 130 soas to substantially form a coherent unit. In one embodiment, thecorrective filter 210 may be designed to take over the functions of thepellicle 140, thereby protecting the surface of the reticle 130 fromairborne particulates and other forms of contamination. In anotherembodiment, the corrective filter 210 may, in addition to the pellicle140, be attached to the reticle 130. Moreover, depending on theapplication, the corrective filter 210 may be attached anterior orposterior to the reticle 130.

[0027] Although a rectangular shape is shown for the corrective filter210 in FIG. 3, it will be understood that the geometric shape of thecorrective filter 210 may be customized to fit virtually any desireddimensions and shapes suitable for the photolithography system 200. Forexample, the corrective filter 210 may have a shape that is a square,oval, elliptical, polygonal, trapezoidal, and other similar geometricshapes. The corrective filter 210 may comprise any material commonlyused to manufacture optical filters in the art, such as fused silica orcalcium fluoride. Given the optical characteristics of the correctivefilter 210, one of several techniques may be employed to manufacture thecorrective filter 210. These techniques include diamond milling,ion-milling, and mechanical polishing.

[0028]FIG. 4 is a flowchart describing the steps of determining theoptical properties of the corrective filter of FIG. 2. To determine theoptical properties of the corrective filter 210, the following methodmay be followed. At block 410, the wafer 170 is exposed to the lightsource, as illustrated in FIG. 1. Generally, it is desirable to includea lens system to compensate for the reticle's magnification (orreduction) effect on the exposed image. In some cases, e.g., if thereticle has no significant distortion on the dimension of the image, itmay be unnecessary to include a lens system when exposing the wafer tothe light source. A measuring instrument, such as a scanning electronmicroscope model number 7830SI manufactured by KLA or 8100XPmanufactured by OPAL, measures one or more image features in theprojected pattern on the photoresist layer 160 (block 420). Some of themeasured features may include positional offsets, distances betweenlines and spaces, line CD's, and contact related features. The lines maybe produced by the pattern 132 in an isolated (e.g., a single line) ordense (e.g., a group of multiple lines) form in the exposure field. Themeasurements may be taken in the horizontal, vertical, sagittal, and/ortangential geometric planes. Depending on the microscope, themeasurements may be taken with varying degrees of resolution, which isoften expressed in picture element (pixel) per unit area (e.g., inch) orpixel density. As is well known in the art, as the resolution increases,the finer and more detailed are the measurements. Hence, if desired, theresolution may be selectively increased to improve the accuracy ofcorrection. More particularly, for fine (e.g., smaller CDs) reticleregions the resolution of measurement may be selectively increased.Similarly, for coarse (e.g., larger CDs) reticle regions the resolutionof measurement may be selectively decreased. Hence, the type ofmeasurements may vary to accommodate various regions of the same reticle130.

[0029] The measurements may be stored in a computer memory, such as ahard disk, for further processing (block 430). Programmed with properinstructions, the computer analyzes the measurements to determine thedistortions or errors across the exposure field (block 440). Forexample, based on the geometric difference between measured and expectedprojected point features, such as point size and positional offset, thecomputer determines the optical deviation (i.e., offset) of theprojected image from that expected by a defect-free reticle. Theinstructions may be programmed using any computer language used in theart, such as C, Fortran, or other similar languages. Alternatively, adedicated processor having instructions in the form of firmware may beused to implement this method. In either case, based on the calculatedoffsets for all measured pixels, the computer determines the opticalcorrection characteristics of the corrective filter 210 (block 450).

[0030]FIG. 5A is a ray diagram of light rays passing through the reticle130 having one or more defects. As shown in FIG. 5A, the reticle 130typically includes a pattern 132 that causes a desired pattern to beprojected in the exposure field on the wafer 170 (not shown in thisfigure). As noted above, as light rays 520 pass through the reticle 130,a defect in the reticle 130 causes a defect in the pattern of theexposed image. For example, the defect may comprise regions 510 a and511 a that are unacceptably narrow or small. The defective regions 510 aand 511 a may be caused by an excess overflow of the layer of thepattern 132. If left uncorrected, the regions 510 a and 511 a causeunacceptable defects (e.g., CD variations) in the exposure field of thewafer 170.

[0031] Accordingly, FIG. 5B shows a ray diagram of light rays passingthrough the reticle 130 after correction by the corrective filter 210 inaccordance with the invention. In this embodiment, the corrective filter210 is placed after the reticle 130, i.e., at a distance from the lightsource 110 (not shown in this figure) that is greater than the distancebetween the reticle 130 and the light source 110. It is desirable toposition and, more particularly, align the corrective filter 210 so asto cause light rays received from the defective regions 510 a and 511 ato diverge, thereby widening the defective regions 510 a and 511 a toregions 510 b and 511 b, respectively. Thus, the corrective filter 210includes corrective elements having optically divergent properties, suchas one or more divergent lens-like regions 510 b and 511 b that areoptically aligned to respectively correct the defective regions 510 aand 511 a to the same or various degree of divergence. In otherembodiments, the corrective filter 210 may include one or moreconvergent lens-like regions (not shown in this figure) that areoptically aligned with defective reticle regions requiring same orvarious degrees of convergence. Additionally, the corrective filter 210may include one or more regions 515 b having a substantially small or nooptical effect on light rays received from reticle regions that are notdefective, or having only minor defects, e.g., the region 515 a.

[0032] Therefore, with its convergent and divergent optical properties,the corrective filter 210 may correct virtually all forms of reticledefects, including deviations in the pattern 132 due to an absence of orexcess in pattern material (e.g., chrome). Moreover, the correctivefilter 210 may correct defects arising from the lens system 150, or fromany other source in the photolithography system 100. Accordingly, insome applications, the corrective filter 210 may compensate for defectswithout having to determine the exact source of the defect.

[0033] In view of the foregoing, it will be appreciated that theinvention overcomes the long-standing need for a correction filter andmethod of correcting defects in individual components of an opticalsystem, such as the reticle. The invention may be embodied in otherspecific forms without departing from its spirit or essentialcharacteristics. The described embodiment is to be considered in allrespects only illustrative and not restrictive. The scope of theinvention is, therefore, indicated by the appended claims rather by theforegoing description. All changes which fall within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A photolithography system comprising: a firstplate having a pattern, the first plate being configured to receivelight from a light source and to project an image through the pattern,the pattern including a defect causing distortion of the light; and asecond plate located in a path of the projected image, the second platecomprising one or more portions configured to diverge or converge atleast a portion of the light distorted by the defect.
 2. The system ofclaim 1, wherein the one or more portions of the second plate aregeometrically related to the defect.
 3. The system of claim 1, whereinthe one or more portions of the second plate are optically aligned withthe defect such that the portion of the light distorted by the defectcan be directed to the one or more portions of the second plate.
 4. Thesystem of claim 1, wherein the one or more portions are characterized bya degree of divergence or convergence that is commensurate with thedegree of distortion of the light.
 5. The system of claim 1, wherein thesecond plate further comprises one or more regions having substantiallyno optical effect on the light directed thereto.
 6. The system of claim5, wherein the one or more regions of the second plate are geometricallyrelated to an area of the pattern that is defect-free.
 7. The system ofclaim 1, wherein the second plate further comprises a pattern thereon.8. The system of claim 1, wherein the first and second plates areattached to each other.
 9. The system of claim 1, wherein the defectcomprises a region that is undesirably transmissive or obstructive tolight.
 10. The system of claim 9, wherein the second plate comprises afilter configured to correct the effect of the defect on the distortedlight.
 11. The system of claim 1, wherein the second plate is configuredto correct the distorted projected image to a desired projected image.12. The system of claim 1, wherein a distance from the light source tothe second plate is greater than a distance from the light source to thefirst plate.
 13. The system of claim 1, wherein the first platecomprises an optically transparent material.
 14. The system of claim 13,wherein the material comprises quartz.
 15. The system of claim 1,wherein the pattern comprises chrome.
 16. The system of claim 1, whereinthe second plate comprises fused silica or calcium fluoride.
 17. Thesystem of claim 1, further comprising a third plate covering a surfaceof the first plate.
 18. The system of claim 17, wherein the third platecomprises an optically transparent material.
 19. The system of claim 17,wherein the pattern is formed on the surface covered by the third plate.20. The system of claim 1, further comprising a wafer coated with aphoto-resist layer and positioned to receive the projected image. 21.The system of claim 20, further comprising at least one lens positionedbetween the first plate and the wafer.
 22. The system of claim 1,further comprising a lens positioned between the light source and thefirst plate.
 23. The system of claim 1, wherein the first platecomprises a reticle, and wherein the second plate comprises a filter.24. The system of claim 23, wherein the filter comprises a pellicle thatis attached to the reticle.
 25. A method of correcting a defect in apattern of a photolithography system, the method comprising: receivinglight from a source by a first plate having a pattern that includes adefect, which distorts light; projecting an image through the pattern;receiving the projected image by a second plate comprising one or moreportions; and diverging or converging at least part of the distortedlight rays by the one or more portions of the second plate.
 26. Themethod of claim 25, further comprising exposing a wafer coated with aphoto-resist layer to an image.
 27. The method of claim 26, wherein theprojected image comprises light rays diverged or converged by the one ormore portions of the second plate.
 28. A method of providing a plateconfigure to correct a defect of a pattern in a photolithography system,the method comprising: projecting an image on a surface through apattern of a first plate, the pattern including a defect causingdistortion of light received by the first plate; obtaining at least onemeasurement of a feature of the image projected on the surface;analyzing the measurement of the feature with reference to a desiredfeature so as to define the defect in the pattern; and defining at leastone property of a second plate comprising one or more portionsconfigured to diverge or converge at least part of the light distortedby the defect.
 29. The method of claim 28, wherein the desired featurecomprises one that can be obtained from a pattern substantially free ofthe defect.
 30. The method of claim 28, wherein the provision of thesecond plate comprises determining based on the analysis opticalcharacteristics of the second plate to substantially compensate thedefect.
 31. The method of claim 28, wherein the provision of the secondplate comprises placing the second plate in a path of the projectedimage.
 32. The method of claim 28, wherein the obtaining at least onemeasurement comprises recording a pixel's offset distance from a desiredlocation.
 33. The method of claim 28, wherein the obtaining at least onemeasurement comprises recording a pixel's offset dimension from adesired size.
 34. A photolithography system comprising: means forprojecting an image on a surface through a pattern of a first plate, thepattern including a defect causing distortion of light received by thefirst plate; means for obtaining at least one measurement of a featureof the image projected on the surface; means for analyzing themeasurement of the feature with reference to a desired feature so as todefine the defect in the pattern; and means for determining opticalcharacteristics of a second plate for substantially compensating for thedefect.
 35. A photolithography system comprising: means for patterninglight to receive light from a light source and to project an imagethrough the pattern, the patterning means including a defect causingdistortion of the light; and means for converging or diverging lightlocated in a path of the projected image, the converging or divergingmeans converging or diverging at least part of the light distorted bythe defect.
 36. The system of claim 35, wherein the converging ordiverging means is attached to the patterning means.
 37. The system ofclaim 35, wherein the converging or diverging means is geometricallyrelated to the location of and configured to compensate for the patterndefect.
 38. The system of claim 35, wherein the converging or divergingmeans accomplishes correcting the pattern defect.
 39. The system ofclaim 35, further comprising means for compensating for an opticaldistortion due to a variation in temperature in the photolithographysystem.